1. Field of the Invention
This invention relates to a low oxygen content polymetalosilazane and to a process of producing same.: The present invention also pertains to a metal-containing silicon nitride ceramic and to a process of preparing same.
2. The Prior Art
Ceramic fibers have been attracting much attention for their favorable properties such as high mechanical strengths and resistance to heat and chemicals and will find use for a variety of applications as reinforcing materials for various composite articles such as engine parts, fan blades and aircraft structures.
U.S. Pat. No. 4,886,860 discloses a process of preparing a polymetalosilazane wherein a polysilazane having a number molecular weight of 100-50,000 and having a skeleton consisting essentially of units of the following general formula: ##STR1## wherein R.sup.1, R.sup.2 and R.sup.3 is independently selected from hydrogen, an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, a group other than the above-mentioned groups in which the atom bonded directly to the silicon atom is a carbon atom, an alkylsilyl group, an alkylamino group and an alkoxy group is reacted with a metal alkoxide of the formula: EQU M(OR.sup.4).sub.n
wherein M is a metal selected from those of groups IIA and III through V of the Periodic Table, R.sub.4 is hydrogen, an alkyl having 1-20 carbon atoms or an aryl and n is the valence of the metal M with the proviso that at least one of the R.sub.4 groups is the alkyl group or aryl group. The polymetalosilazane obtained by the above method unavoidably contains oxygen derived from the metal alkoxide. The oxygen introduced into the polymer is converted into SiO gas during the calcining stage so that small pores or gaps are apt to be formed in the ceramic product, which adversely affects the mechanical strengths and heat resistance of the ceramic.
U.S. Pat. No. 4,730,026 (equivalent of EP-A-262,914) discloses a polymetalosilazane which is obtained by reacting a silazane with an oxygen-free metal compound such as an alkyl metal or a metal halide and in which the silazane is cross-linked with, for example, --MR-- linkages. a ceramic body obtained therefrom. The polymetalosilazane obtained using the oxygen free metal compound gives a ceramic having a low oxygen content. However, the above method is not practical and requires much costs because the metal compound is dangerous.